[1][2] The Workers' Revolutionary Party (PRT) had refused to join the merger.

[4] The party participated solely in the 1988 elections, in which it had postulated Heberto Castillo as its presidential candidate.

The party registered Castillo as their nominee on 30 June 1987 after holding internal elections.

[5] The following month, it was announced that a faction of the PST led by Graco Ramírez would join the PMS as its sixth integrant.

[6] However, a month before the elections, Castillo withdrew his candidacy in support of a unified Socialist coalition behind Cuauhtémoc Cárdenas Solórzano of the National Democratic Front.